Adsorption processes are widely used in industry for separation of fluid mixtures. This separation is based on preferential sorption of selective components on the surface or within the cavities of sorbent materials. For most separation systems, the adsorbent material has a large surface area to provide reasonable adsorptive capacities. The commonly used adsorbents, such as molecular sieve zeolites, activated carbon, alumina, and silica gel, have surface areas of at least 200 m2/g.
Many industrial adsorption processes are carried out in fixed-bed type columns. The adsorbent material (e.g., granules, particles) are generally packed and immobilized in a cylindrical vessel. As the fluid mixture designated for separation is passed through the packed column, the adsorbable components in the mixture are taken up and retained by the adsorbent as the adsorbate, and the non-adsorbable components pass through the column via the void spaces among the adsorbent granules.
Given that the adsorbent has a limited adsorption capacity, the adsorbent will gradually become saturated with adsorbate, and periodic adsorbent regeneration is required. For continuous processing of a feed fluid mixture, a multi-bed system is used in which each bed goes through the adsorption/regeneration cycle in sequence. Several different regeneration methods have been used commercially, including a pressure swing adsorption (PSA) process and a thermal swing adsorption (TSA) process. In the TSA process, the saturated adsorbent is regenerated by purging with a hot gas. Each heating/cooling cycle usually requires a few hours to over a day. In the PSA process, adsorbent regeneration is effected by purging with a portion of the purified product gas at reduced pressure. The throughput in PSA is generally higher than that of the TSA, since faster temporal cycles, usually in minutes, are possible.
Apart from the adsorptive capacity of the adsorbent, the adsorption rate and pressure drop are two important factors that must be considered in adsorbent column design. Pressure drop through the adsorbent column should be minimized, because high fluid pressure drop can cause movement or fluidization of the adsorbent particles, resulting in serious attrition and loss of the adsorbent. The adsorption rate has a significant bearing on the efficiency of the adsorption process. This rate is usually determined by the mass transfer resistance to adsorbate transport from the bulk fluid phase to the internal surfaces of the adsorbent particles. A slow adsorption rate, due to large mass transfer resistance, will result in a long mass transfer zone (MTZ) within which the adsorbent is only partially saturated with adsorbate. The adsorbent in the region upstream of the MTZ is substantially saturated with adsorbate, while that downstream of the MTZ is essentially free of adsorbate. As the fluid continues to flow, the MTZ advances through the adsorber column in the direction of the fluid stream. The adsorption step must be terminated before the MTZ reaches the adsorber outlet in order to avoid the breakthrough of adsorbate in the effluent stream. A long mass transfer zone, which contains a large quantity of partially utilized adsorbent, will, therefore, result in a short adsorption step and inefficient use of the adsorbent capacity.
Both the pressure drop and the mass transfer resistance are strongly influenced by the size of the adsorbent particles. Changing the particle size, unfortunately, has opposite effects on these two important factors. The interstitial space between the adsorbent particles in the fixed-bed is proportional to the size of the particles. Since the resistance to the fluid flow through the adsorber is inversely proportional to the pore size of the packed bed, the use of small adsorbent particles will cause a high pressure drop. For this reason, the sizes of particles of commercial adsorbents for fixed-bed operation are generally larger than 2 mm in average diameter.
In addition, almost all the surface areas of commercial adsorbents are located at the interior of the adsorbent particle. For adsorption to occur, the adsorbate needs to be transported from the external fluid phase to the interior surface of the particle. The transport rate is influenced by two mass transfer mechanisms in series: (a) interfacial mass transfer—diffusion through the fluid boundary layer surrounding the external surface of the adsorbent particle; and (b) intraparticle mass transfer—diffusion through the internal pore space (micropores and macropores) of the particle to its interior surface where adsorption takes place. The size of the particle has significant effects on the rates of these two diffusion processes. Small particles offer large fluid/solid contact areas in the fixed bed for interfacial mass transfer and reduce the path length for the intraparticle diffusion. Hence, small adsorbent particles will increase the adsorption rate and result in a narrow mass transfer zone for fast and efficient operation of adsorption/desorption cycles. Thus, small adsorbent particles are desirable for efficient adsorption processes, but the minimum particle size is limited by acceptable hydrodynamic operating conditions of the fixed bed adsorber. That is, one wants to avoid fluidization and excessive pressure drop.
Accordingly, there is a need for compositions and methods of adsorbing at least a component of a medium characterized by a relatively small particle size and yet still able to operate with an acceptable pressure drop. It is to the provision of such compositions and methods that the various embodiments of the present invention are directed.